Abstract-Cannabinoids, including the endogenous ligand anandamide (arachidonyl ethanolamide), elicit pronounced hypotension in rats via activation of peripherally located CB1 cannabinoid receptors, which have been also implicated in endotoxin (lipopolysaccharide [LPS])-induced hypotension. The present study was designed to test the role of vascular CB1 receptors in cannabinoid-and endotoxin-induced mesenteric vasodilation. In the isolated, buffer-perfused rat mesenteric arterial bed precontracted with phenylephrine, anandamide induced long-lasting (up to 60 minutes) dose-dependent vasodilation (ED 50 : 79Ϯ3 nmol; maximal relaxation: 77Ϯ2%), inhibited by 0.5 to 5.0 mol/L of the selective CB1 receptor antagonist SR141716A. Low doses of the calcium ionophore ionomycin also caused mesenteric vasodilation inhibited by SR141716A. The metabolically stable analogue R-methanandamide elicited mesenteric vasodilation (ED 50 : 286Ϯ29 nmol), whereas the potent synthetic CB1 receptor agonists WIN 55212-2 and HU-210 caused no change in vascular tone or only a minor dilator effect not affected by SR141716A, respectively. The endogenous ligand 2-arachidonyl glycerol caused no change in vascular tone, whereas ⌬ 9 -tetrahydrocannabinol and arachidonic acid caused mesenteric vasoconstriction. After endothelial denudation, the dilator response to anandamide was slightly reduced and was no longer inhibited by SR141716A. In preparations from LPS-pretreated rats, SR141716A alone caused a significant and prolonged increase in perfusion pressure, whereas it had no such effect in control preparations perfused in vitro with or without LPS or after endothelial denudation in preparations from rats pretreated with LPS. We conclude that anandamide-induced mesenteric vasodilation is mediated by an endothelially located SR141716A-sensitive "anandamide receptor" distinct from CB1 cannabinoid receptors and that activation of such receptors by an endocannabinoid, possibly anandamide, contributes to LPS-induced mesenteric vasodilation in vivo. Key Words: vasodilation Ⅲ cannabinoids Ⅲ anandamide Ⅲ endotoxin E ndogenous cannabinoids (or endocannabinoids) represent a novel class of lipid ligands that share receptor binding sites with plant-derived cannabinoids, such as ⌬ 9 -tetrahydrocannabinol (THC), and that mimic the neurobehavioral effects of the latter. 1 Two receptors have been identified by molecular cloning that can recognize cannabinoids with high affinity: the CB1 receptor is present primarily in the brain 2 but also in some peripheral tissues 3,4 and the CB2 receptor is expressed by cells of the immune system. 5 The mRNA of a splice variant of the CB1 receptor, CB1A, has also been identified. 3 Two endocannabinoids have been characterized in some detail: arachidonyl ethanolamide, or anandamide, 6 and 2-arachidonyl glyceride, or 2-AG. 7,8 Neurons in the brain can synthesize anandamide 9 and 2-AG, 10 and both substances have been shown to influence central neural functions, such as long-term potentiation. 10,11 Peripheral tissues also contain ...